The proposal is designed to develop an understanding of the mechanisms and cytology of early female mammalian meiosis. It is planned to investigate (1) the number and location of chiasmata in diplotene oocytes from fetal material of 5 species to answer fundamental questions about terminalization and ageing effects on disjunction, (2) to develop a technique for banding of early female meiotic stages suitable for specific identification of each bivalent, (3) to develop pachytene maps and analyses of the major structural rearrangements reported thus far in mammals and (4) to study the mechanism of the switch from mitosis to meiosis in the female fetal gonad and its subsequent inhibition using an in vitro system. BIBLIOGRAPHIC REFERENCES: Wolgemuth-Jarashow, Debra J., Jagiello, Georgia M., Henderson, Ann S. Localization of rDNA in small, nucleolus-like structures in human oocyte nuclei. Human Genetics, 36:63-68, 1977. Hembree, W.C., Fang, J.S. and Jagello, G. Meiotic abnormalities in male reproductive dysfunction. The Testis in Normal and Infertile Men, edited by Philip Troen and Howard R. Nankin, Raven Press, New York 1977, pp. 25-33.